1. Field of Invention
This invention relates to cooling systems. Specifically, the present invention relates to cryogenic cooling systems for cooling focal plane arrays.
2. Description of the Related Art
Cryogenic cooling systems are employed in various demanding applications including military and civilian active and remote sensing, superconducting, and general electronics cooling. Such applications often demand efficient, reliable, and cost-effective cooling systems that can achieve extremely cold temperatures below 80 degrees Kelvin.
Efficient cryogenic cooling systems are particularly important in sensing applications involving high-sensitivity infrared focal plane arrays of electromagnetic energy detectors (FPA""s). An FPA may detect electromagnetic energy radiated or reflected from a scene and convert the detected electromagnetic energy into electrical signals corresponding to an image of the scene. To optimize FPA imaging performance, any FPA detector nonuniformities, such as differences in individual detector offsets, gains, or frequency responses, are corrected. Any spatial or temporal variations in temperature across the FPA may cause prohibitive FPA nonuniformities.
FPA""s are often employed in missile targeting applications, where weight, size, and spatial and temporal uniformity of cryogenic cooling systems are important design considerations. An FPA must operate at stable cryogenic temperatures for maximum performance and sensitivity.
Conventionally, a cooling fluid is applied to the FPA via a cooling interface. Heat is transferred to the cooling fluid from the FPA. The heated fluid is then expelled from the missile or re-cooled via a heat exchanger integrated into the FPA. The cooling fluid requires a heavy and bulky FPA cooling interface and heat exchanger, which are attached to the FPA mounting assembly. Consequently, the FPA assembly must have additional mechanical support to secure the interface, heat exchanger, and cooling fluid. The bulky components and additional support hardware may require additional cooling, which increases demands placed on the cooling system. The bulky support structure, conventionally thought to improve temperature stability, may conduct excess heat from the warm missile body into the FPA, thereby reducing system cooling efficiency. Furthermore, the additional bulky mechanical FPA support hardware may cause alignment problems with the on board optical or infrared system during installation and operation, thereby increasing installation and operating costs. In addition, missile maneuvering may cause the cooling liquid to slosh in the cooling interface, creating undesirable temperature instabilities.
Alternatively, Joule-Thompson cycle coolers are employed. A Joule-Thomson cycle cooler typically applies a regulated flow of cold gas over the infrared FPA. However, Joule-Thompson cycle coolers require undesirably expensive and bulky compressed gas canisters that must remain on the missile, aircraft, or other system. The additional weight increases the overall operating costs and reduces maneuvering capability and range of the accompanying system. Furthermore, excessive shock or vibration environments from missile maneuvering may interrupt gas flow, thereby creating potentially prohibitive temperature instabilities, resulting in reduced missile performance.
To address size and cost issues associated with using gas canisters, compressors, or other heat exchangers, more advanced construction materials are under continual development. In addition, researchers are attempting to design FPA""s with reduced cooling requirements. Unfortunately, this has matured slowly and does not promise satisfactory solutions for high performance applications in the foreseeable future.
Hence, a need exists in the art for an efficient cryogenic cooling system for uniformly cooling an infrared FPA. There exists a further need for a cryogenic cooling system that efficiently employs a solid cryogen to cool an FPA with minimal weight and size impact.
The need in the art is addressed by the cryogenic cooling system for cooling electromagnetic energy detectors of the present invention. In the illustrative embodiment, the inventive system is adapted to cool infrared focal plane arrays. The system includes a first mechanism for accommodating cryogen fluid in one or more spaces. A second mechanism freezes the cryogen fluid in the one or more spaces adjacent to the electromagnetic energy detectors.
In a more specific embodiment, the electromagnetic energy detectors comprise one or more focal plane arrays. The second mechanism includes a heat exchanger that is mounted separately from the first mechanism. The one or more spaces are fitted with three-dimensional cooling interface surfaces. The first mechanism includes a solid cryogen reservoir having a thermally conductive matrix for implementing the three-dimensional cooling surfaces. The thermally conductive matrix is a copper, graphite, or beryllium matrix, and the solid cryogen reservoir is a beryllium reservoir.
The solid cryogen reservoir includes one or more mounting features for mounting the reservoir and has a surface for mounting the focal plane array on the reservoir. The second mechanism includes a mechanism for employing the Joule-Thomson effect (also called the Joule-Kelvin effect) to cool the cryogen fluid to a liquid state. The first mechanism includes a selectively detachable cryogen canister for providing pressurized cryogen fluid to the heat exchanger.
In an illustrative embodiment, the heat exchanger outputs cooled cryogen gas to plural solid cryogen reservoirs to cool plural corresponding infrared focal plane arrays. The cryogenic cooling system is mounted on or within a missile system. The cryogenic cooling system is connected to a cryogen canister and a heat exchanger for providing the cryogen fluid to a cryogen reservoir with three-dimensional cooling surfaces. A Joule-Thomson orifice employs the Joule-Thomson effect to create the cryogen fluid output from the heat exchanger.
The heat exchanger, which is positioned separately from the reservoir, employs a conduit to direct the fluid to the cryogen reservoir. An additional mechanism selectively detaches the gas canister and/or the heat exchanger from the missile after a predetermined amount of the fluid is collected within the cryogen reservoir or after a predetermined time interval.
The novel design of the present invention is facilitated by the second mechanism, which freezes cryogen in a cooling interface adjacent to a focal plane array. Freezing the cryogen enables remote positioning of the heat exchanger relative to the cooling interface. The cooling interface and accompanying focal plane array assembly no longer require mounting of the heat exchanger in the same assembly to increase the temperature stability of the focal plane array. The frozen cryogen in combination with the efficient solid cryogen cooling interface of the present invention provides sufficient temperature stability. Consequently, costs, cooling inefficiencies, and sensor alignment problems associated with conventional cooling systems are avoided.